This is a randomized, double-blind treatment of the putative neuroprotective effects of Allopurinol on the developing brain exposed to hypoxic-ischemic insults. Infants undergoing cardiac surgery which requires circulatory arrest are treated with either Allopurinol or a placebo prior to surgery. The major outcome measures are acute encephalopathy (including coma and seizures), and death. Infants undergo neurological examination and developmental assessment in follow-up visits up to two years of age. It has been known that hypoxic-ischemia is a potent casue of cell death and tissue necrosis. The exact biochemistry of injury is still under intense investigation. One of the most fascinating discoveries in this line of research is that aberrant oxygen metabolism plays a role in tissue injury. There is a growing body of evidence which has identified oxygen free radical formation as an important biochemical cascade operative in the ultimate production of cellular insult. This model serves to provide a potential therapuetic mechanism of intervention that might prevent or ameliorate the consequences of hypoxia reperfusion injury. Allopurinol may prevent the formation of free radicals in post ischemic tissues. Enrollment of study subjects was completed on September 15, 1997. A total of 350 patients were enrolled.